Coating apparatus – Gas or vapor deposition – Work support
Reexamination Certificate
2002-03-15
2004-08-17
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
Work support
C118S724000, C118S725000, C118S500000, C156S345510, C156S345520, C156S345550
Reexamination Certificate
active
06776849
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to supports for wafers in semiconductor processing chambers and, more particularly, to a wafer holder for supporting a wafer within a cold wall chemical vapor deposition chamber.
2. Description of the Related Art
High-temperature ovens, or reactors, are used to process semiconductor wafers from which integrated circuits are made for the electronics industry. A substrate, typically a circular silicon wafer, is placed on a wafer holder. If the wafer holder helps to attract heat, it is called a susceptor. The wafer and wafer holder are enclosed in a quartz chamber and heated to high temperatures, such as 600° C. (1112° F.) or higher, by a plurality of radiant lamps placed around the quartz chamber. A reactant gas is passed over the heated wafer, causing the chemical vapor deposition (CVD) of a thin layer of the reactant material on the wafer. Through subsequent processes in other equipment, these layers are made into integrated circuits, with a single layer producing from tens to thousands of integrated circuits, depending on the size of the wafer and the complexity of the circuits.
If the deposited layer has the same crystallographic structure as the underlying silicon wafer, it is called an epitaxial layer. This is also sometimes called a monocrystalline layer because it has only one crystal structure.
Various CVD process parameters must be carefully controlled to ensure the high quality of the deposited films and the resulting semiconductor. One such critical parameter is the temperature of the wafer during the processing. The deposition gas reacts at particular temperatures and deposits on the wafer. If the temperature varies greatly across the surface of the wafer, uneven deposition of the reactant gas occurs. Similarly, temperature uniformity can be important for a variety of other semiconductor fabrication processes, such as etching, annealing, doping, etc.
Rotatable wafer holders are known in the art. Rotation of the wafer holder results in more uniform temperature distribution and deposition across the wafer.
In recent years, single-wafer processing of larger diameter wafers has grown for a variety of reasons including its greater precision as opposed to processing batches of wafers at the same time. Although single-wafer processing by itself provides advantages over batch processing, control of process parameters and throughput remains critical. In systems in which the wafer is supported in intimate contact with a large-mass, slab-like susceptor, the necessity of maintaining uniform susceptor temperature during heat-up and cool-down cycles limits the rate at which the temperature could be changed. For example, in order to maintain temperature uniformity across the susceptor, the power input to the edges of the susceptor had to be significantly greater than the power input to the center due to the edge effects.
As explained above, CVD processing often occurs at temperatures of 600° C. (1112° F.) or higher. One common problem associated with CVD processing is that, when a cold wafer is loaded onto the top surface of a susceptor inside a pre-heated reaction chamber, the wafer tends to experience “thermal shock” due to thermal gradients within the wafer from sudden conductive heat transfer from the hot susceptor to the cold wafer. These thermal stresses can result in wafer “curl” and “pop,” as well as damage to the backside of the wafer. The largest problem associated with such thermal gradients is wafer pop, which causes the wafer to move randomly on the susceptor surface. This movement causes temperature non-uniformities, which reduces the repeatability of process characteristics such as thickness uniformity.
One method to reduce the problems associated with thermal shock is to substantially decrease wafer load temperatures. This is not common because it adversely affects throughput, since the temperature must be decreased before each new wafer is loaded and then increased before processing of the wafer can begin. Decreases in throughput results in decreased production and greater manufacturing costs. Thus, in order to maintain a desired throughput, some degree of wafer curl and pop are usually tolerated.
Some susceptors are equipped with vertically oriented lift pins that are vertically moveable through holes in the surface of the susceptor upon which the wafer rests. When the lift pins are elevated, the wafer is separated from the susceptor surface so as to slow heat transfer from the susceptor to the wafer. This permits the wafer to preheat, thus reducing thermal gradients when the wafer is lowered into contact with the susceptor. When the lift pins are lowered, the wafer is brought into flush contact with or very close to the susceptor surface, permitting conductive heat transfer therebetween.
Presently, there is a need for an improved wafer support system, which permits higher wafer load temperatures while avoiding the problems associated with thermal shock.
SUMMARY OF THE INVENTION
It is a principle object and advantage of the present invention to provide an improved wafer support system that permits a cold wafer to be loaded into a pre-heated reaction chamber while avoiding the above-mentioned problems associated with thermal shock.
Prior art susceptors utilizing lift pins can reduce the effects of thermal shock by permitting the loading of a wafer onto the lift pins in their elevated positions. When the wafer is in the elevated position, the wafer temperature can be permitted to increase much more gradually than would occur if the wafer were immediately brought into flush contact with a flat surface of the susceptor. In order to reduce the risk of thermal shock, the wafer can be maintained in the elevated position until the wafer temperature increases to a degree such that the likelihood of thermal shock is substantially reduced or eliminated when the wafer is eventually lowered onto the susceptor surface.
One problem with lift pins is that they tend to scratch the lower surface of the wafer, which in turn increases the likelihood of crystallographic slip. Slip is a defect in the crystalline structure of the wafer, which destroys any devices through which it may pass. The presence of scratches on a wafer causes slip to occur in the wafer at lower temperatures than if no scratches are present. In other words, the presence of scratches makes a wafer less robust and less able to tolerate high temperatures. Scratches also increase the susceptibility of a wafer to slip under rapidly varying temperature conditions. In addition, scratches cause nodule growth on the backside of the wafer, which leads to alignment problems during photolithography.
The preferred embodiments of the present invention solve this problem by providing a vertically moveable lift ring that supports the outer radial periphery of a wafer. The lift ring can be raised above the remaining portions of the susceptor to receive a newly loaded wafer. In the raised position, the only heat conduction received by the wafer is via the peripheral lift ring. Heat conduction between the remainder of the susceptor and the raised wafer is substantially prevented. Moreover, heat conduction to the wafer is localized at the wafer periphery, preferably in the exclusion zone of the wafer. When the wafer is in the raised position, the majority of the heat received by the wafer is in the form of (1) radiation from the heated susceptor (from both the lift ring and the remainder of the susceptor), (2) radiation from the heat lamps, if they are on, and (3) convection from warm gas within the chamber. In addition, as indicated above, some heat is received at the wafer edge in the form of conduction from the lift ring. The wafer can be maintained in the raised position until the wafer temperature rises to a level sufficient to substantially prevent or significantly reduce thermal shock to the wafer when the wafer is eventually lowered onto the remainder of the susceptor. The lift ring provides more stable support to the wafer than lift pins
Aggarwal Ravinder K.
Goodman Matthew G.
Keeton Tony J.
ASM America Inc.
Kackar Ram N
Knobbe Martens Olson & Bear LLP
Mills Gregory
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